SN1 Mechanism

The SN1 mechanism is similar to SN2 in that you get a substitution product, but the path to get there is completely different. It’s important that we understand how it’s different from SN2

Summary: A neutral nucleophile reacts with an inaccessible leaving group to produce substitution in two-step.

The slow step of this mechanism is the formation of a carbocation intermediate. These types of intermediates are unstable, so anything that we can do to stabilize them will help them form faster.

-R groups stabilize carbocations through a phenomenon called hyperconjugation. Meaning that the more substituted the carbocation, the more stable it is. 

Concept: Understanding the properties of SN1

9m

You are a manager at Pepe and Son Carbocations Inc, and your boss has asked you to increase production of your product (carbocations in a box- these are awesome).

You have plenty of boxes, but it takes time for the conveyer belt to crank out these custom carbocations.

  • Will increasing the number of boxes increase the amount of product?
  • Will increasing the speed of the conveyer belt increase the amount of product? 

Properties of SN1 reactions:

  • Nucleophile =  Weak
  • Leaving Group =  Highly Substituted
  • Reaction coordinate = Intermediate
  • Reaction = Two-Step
  • Rate =  Unimolecular
  • Rate =  k[RX]
  • Stereochemistry = Racemic
  • Nickname = Solvolysis

Problem: Predict the product of the following reaction:

 

11m

The product must be a racemate if the original leaving group is located on a chiral center.

NOTE: Substitution reactions with neutral nucleophiles require an additional deprotonation step. 

Problem: Predict the product of the following reaction:

 

5m